When I try to do anything with JRuby 1.1.6 and Java 5, I get this error:

Error: Some library (perhaps JRuby) was built with a later JVM version.
Please use libraries built with the version you intend to use or an
earlier
one.
Specify -w for full UnsupportedClassVersionError stack trace

Are you sure this was the released version of JRuby 1.1.6 and not the
release candidate? We had an issue during the RC that had a Java 6
compiled version of jna-posix, but that was corrected for final
release.